You can also take a look to Web Resource Optimizer 4 J (WRO4J) which allows a lots a things (minification, resource merging) and supports Less CSS (at runtime as far as I know).
This means: you add wro4j filter to your web.xml and when you ask for a CSS, your .scss (or .saas) files are compiled to standard CSS.
I have not used it yet, but it seems more advanced than other products listed here.
In fact, I was reading comments on the Less for Java website (http://www.asual.com/lesscss/) and WRO4J uses this library to provide it's "on the fly Less compilation". So I think Less for Java is the way to go.
